Bolometers are energy detectors based upon a change in the resistance of materials (called bolometer elements) that are exposed to a radiation flux. The bolometer elements have been made from both metals and semiconductors. In case of the metals, the resistance change is essentially due to a variation in the carrier mobility, which typically decreases with temperature. In contrast, greater sensitivity can be obtained in high-resistivity semiconductor bolometer elements wherein the free-carrier density is an exponential function of temperature; however, thin film fabrication of semiconductor elements for the construction of bolometers is a difficult task.
In FIGS. 1 and 2, there are shown a perspective view and a cross sectional view illustrating a three-level bolometer 100, disclosed in U.S. application Ser. No. 09/102,364 entitled "BOLOMETER HAVING AN INCREASED FILL FACTOR". The bolometer 100 comprises an active matrix level 110, a support level 120, at least a pair of posts 170 and an absorption level 130.
The active matrix level 110 has a substrate 112 including an integrated circuit (not shown), a pair of connecting terminals 114 and a protective layer 116. Each of the connecting terminals 114 made of a metal is located on top of the substrate 112. The protective layer 116 made of, e.g., silicon nitride (SiN.sub.x), covers the substrate 112. The pair of connecting terminals 114 are electrically connected to the integrated circuit.
The support level 120 includes a pair of bridges 140 made of silicon nitride (SiN.sub.x), each of the bridges 140 having a conduction line 165 formed on top thereof. Each of the bridges 140 is provided with an anchor portion 142, a leg portion 144 and an elevated portion 146, the anchor portion 142 including a via hole 152 through which one end of the conduction line 165 is electrically connected to the connecting terminal 114, the leg portion 144 supporting the elevated portion 146.
The absorption level 130 is provided with a serpentine bolometer element 185 made of titanium (Ti), an absorber 195 made of silicon nitride (SiN.sub.x) and an IR absorber coating 197 formed on top of the absorber 195. The absorber 195 is fabricated by depositing silicon nitride before and after the formation of the serpentine bolometer element 185 to surround the serpentine bolometer element 185.
Each of the posts 170 is placed between the absorption level 130 and the support level 120. Each of the posts 170 includes an electrical conduit 172 made of a metal, e.g., titanium (Ti), and surrounded by an insulating material 174 made of, e.g., silicon nitride (SiN.sub.x). Top end of the electrical conduit 172 is electrically connected to one end of the serpentine bolometer element 185 and bottom end of the electrical conduit 172 is electrically connected to the conduction line 165 on the bridge 140, in such a way that both ends of the serpentine bolometer element 185 in the absorption level 130 is electrically connected to the integrated circuit of the active matrix level 110 through the electrical conduits 172, the conduction lines 165 and the connecting terminals 114. When exposed to infra-red radiation, the resistivity of the serpentine bolometer element 185 changes, causing a current and a voltage to vary, accordingly. The varied current or voltage is amplified by the integrated circuit, in such a way that the amplified current or voltage is read out by a detective circuit (not shown).
There are certain deficiencies associated with the above described three-level bolometer 100. When selecting the material for the absorber 195, it is important to consider the fabrication conditions, e.g., deposition-temperature, and the material characteristics, e.g., heat-conductivity. In the above described three-level bolometer 100, since silicon nitride (SiN.sub.x) can be formed only at a relatively high temperature, e.g., over 850.degree. C., titanium (Ti) constituting the serpentine bolometer element 185 gets easily oxidized during the formation of the absorber 195, which will, in turn, detrimentally affect the temperature coefficient of resistance (TCR) thereof. Further, silicon nitride (SiN.sub.x) has a relatively high heat-conductivity, reducing the thermal isolation effect of the absorber 195 in the bolometer 100.